Mode-locked lasers form the foundation of a large number of applications ranging from telecommunications, spectroscopy and metrology to nonlinear optics. As active mode-locking is limited to electronic frequencies and involves a high degree of complexity, the ability to use passive mode-locking is desirable to reduce complexity and enable higher repetition rates. Monolithically integrated semiconductor lasers usually use semiconductor saturable absorbers (SESAMS) exploiting colliding pulse mode-locking [1] or compound cavity mode-locking methods [2] to generate short pulses in the femtosecond regime. Within the framework of fiber laser systems, while SESAMS can be used [3], nonlinear optical effects are usually exploited to artificially achieve passive mode-locking. Among these approaches, nonlinear amplifying loop mirrors (NALM) [4,5], nonlinear polarization rotation (NPR) [6,7], Kerr lenses [8], additive pulse mode-locking [9] or four-wave mixing [10] are commonly implemented.
Semiconductor optical amplifier-based laser exploiting nonlinear amplifying loop mirrors (NALM) have been presented [11, 12, 13], with the aim of reducing the input power and push forward integration. These systems usually have average optical powers between about 1 and 10 mW and emit strongly phase-modulated (“chirped”) pulses with duration between about 0.5 ns and 10 ns.
However, there is no such laser system able to directly generate transform-limited pulses, i.e. pulses having a spectral width inversely proportional to the minimal pulse duration, which is highly important for multiplexing applications in telecommunication systems or optical interconnects for example.
Mode-locking based on nonlinear optical interactions such as nonlinear amplifying loop mirrors (NALM) or nonlinear polarization rotation (NPR) is intrinsically related to the nonlinear length of the element, proportional to the length of the element (L), its nonlinear coefficient (y) and the instantaneous optical power (P) in the element, i.e. a product of all three terms (L*·y·*P). As a result, the nonlinear interaction required to achieve passive mode-locking either relies on high optical powers, which are usually obtained through ultra-short pulses (50 fs-10 ps), or from nonlinear interaction over long distances within the element, intrinsically limiting the repetition rate of the laser source. Mode-locking of long bandwidth-limited pulses (>100 ps) finds broad demand in applications requiring both low pump powers and a high-degree of integration, and is very challenging to achieve considering standard passive mode-locking methods.
There is still a need in the art for a passive mode-locked laser system and method.